I've developed an Android application that is used as a Car Home replacement. One of the pieces of information that I return to the user is what I originally called "altitude": the height above sea level the user is currently at while driving.

However, a user sent me an email regarding my use of the word altitude and recommended that I switch it to elevation saying "Altitude is a measurement used to show how far off the ground you are, elevation is used to show how high above sea level you are." I started to believe that what he told me was correct, but after a search I found this interesting comparison of the two. Additionally, Android's API declares this value as "altitude" when returning it from the GPS receiver. Now I'm just confused.

In the US, car drivers are used to road signs like "Elevation 6000 ft" placed sometimes on mountain roads, just to mark a point of specific height. That's why they may think that 'elevation' is more appropriate.
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ArtemMar 17 '11 at 19:47

7 Answers
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Both altitude and elevation are measures of the height of a point relative to some datum. The differences are in how they are derived and what they are normally used for.

Altitude is typically only used to describe the height of an aircraft in flight. It is a barometric measurement expressed relative to the height of a runway or mean sea level in a given location or region (taking into account current local atmospheric conditions), or to an arbitrary standard datum (to eliminate the effect of localised variations in air pressure).

Elevation is usually used to describe the height of the ground, or a feature fixed to the ground. It is a geometric measurement expressed relative to the mean sea level datum established for the region by the national mapping agency.

I would take some care to understand the meaning of a value named altitude obtained from a GPS receiver. At a basic level, GPS can only tell you the receiver’s height above the WGS‑84 ellipsoid, and it cannot be assumed that the surface of the ellipsoid is the same as mean sea level in any given location. Some receivers contain look-up tables to calculate the offset between the two, given a lat/long position, which ought to be incorporated in the determination of ‘altitude’.

Whether to use altitude or elevation for your application is largely a matter of preference, I’d say. Since a moving car is neither an aircraft in flight nor fixed to the ground, I’ve conveniently excluded it from my definitions of the two terms here. As your measurement is geometric rather than barometric, I’d lean towards elevation. Alternatively, you could just keep the terminology simple (if imprecise) and call it height…

The problem with using altitude is that it is a relative measure with a non-standard reference point. Or as wikipedia puts it:

As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object

Since you're not declaring what your reference point is (sea level, ground level, etc.) it's ambiguous at best. Elevation tends not to have this problem, as while it's a relative measure, it's reference point is usually the Mean Sea Level.

Also from WP:

Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.

If you'd prefer to stick with altitude, you could add the modifier True to indicate that you're referring to the altitude above sea level.

I don't think this answer helps much. Both altitude and elevation are relative measures. And while I note its mention on Wikipedia, I've never come across the term true altitude.
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Brian NixonMar 17 '11 at 18:38

@Brian Nixon - Both are relative, but the reference for altitude varies depending on the type of altitude we're talking about. Elevation varies much less commonly (it can be distance from the center of the earth, but that's extremely rare in my experience, with it being almost always above MSL (or the geoid representing MSL)). And for True Altitude how about a NASA reference: mtp.jpl.nasa.gov/notes/altitude/AviationAltiudeScales.html
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DustyMar 17 '11 at 21:38

@Dusty: Noted, and your edit has certainly clarified your point, but to my mind altitude is always a barometric measurement, so including that word in a geometric measurement is potentially confusing. For clarity I would refer to this “true altitude” as height above mean sea level.
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Brian NixonMar 17 '11 at 22:29

@Brian Nixon - I disagree that the word altitude has any intrinsic relation to a barometric measurement other than the fact that it is frequently calculated that way in certain domains. It may be very common, but would you suggest that planes using radar altimeters instead of barometric ones aren't actually calculating their altitude? What about the space shuttle? Is its altitude incalculable?
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DustyMar 17 '11 at 23:31

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@Brian Nixon - I think the fact that it's extremely common in aviation that an unqualified altitude refers to a barometric measurement only implies that it has that meaning in aviation. In other areas which use the term altitude but don't commonly measure it using barometric pressure (let's say something that uses GPS), you contend that they're all actually thinking of it as a substitute for a true barometric pressure altitude?
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DustyMar 18 '11 at 2:13

The other answers have covered a lot of this, but I'd like to add another data point that I think is relevant:

In the context of mountaineering, where people generally wish to talk about their height above sea level (since their height above ground level is a mostly constant and very much less impressive value), the term altitude is used.

I think either one is a completely reasonable choice when referring to a car's height above sea level. Either will be well understood by the user.

I do think there are some contexts in which one or the other is used more frequently, perhaps out of pedantry, but probably mainly out of convention. For example, the height at which a plane is flying is almost exclusively referred to as its altitude. (And I don't believe planes fly higher over mountains to maintain a constant separation from the ground, but rather maintain a steady height above sea level.) Ground-based heights (as for mountain peaks or cities) are often referred to as elevations, though both terms are common for those cases, which is why I believe either could apply to a car.

There is one particular difference I can pin-point and others should correct me if I'm wrong.

Whereas
Altitude is the height of an object above the sea or the ground level,
Elevation is the height of an object with respect to any given level, esp. sea level.
In that sense, Elevation is more generic. So when you are describing a difference (gain or loss) of height, you may want to use the term Elevation instead of Altitude

There is another flaw in using Elevation , minuscule as it may be.

Say you are at an altitude of 15 feet and you are not specifying the units, say ft.. Now if you say the Elevation is 15, it could also mean some sort of an angle with the horizontal, as it can be interpreted as 15 degrees. This by the way is another meaning of the word elevation.

Though chances of this happening are remote and it's possibility can be avoided in totality, yet this might be thin line of difference you are considering while selecting from the two largely interchangeable terms Altitude and Elevation.

Altitude should be used, although it is a relative term and must be qualified to be above sea level (ASL) to be an absolute. Elevation can be an angle from a specified point; an example might be a telescope where to look at a particular point, you set an angle of elevation in degrees.